A cocktail of a plant protein and a common insecticide may be more
lethal to pests than either ingredient used alone
A cocktail of a plant protein and a common insecticide may be more
lethal to pests than either ingredient used alone, according to a new study.

The study, by researchers at Penn State University, found that the
double whammy also inhibits the growth rate of insects and reduces their
chance of developing resistance.

?We found a synergistic effect, where the two insecticides together
decreased the growth rate of caterpillars more than either one did alone,?
said Dawn Luthe, who led the study, which appears in the current issue of
PLoS ONE.

?The insect mortality rate was also much greater than the sum of
mortality rates when only one insecticide was used.?

One of those insecticides, Bt, is commonly used around the world. When
insects feed on plants genetically modified to produce Bt, the toxin binds
to chemical receptors lining the insects? mid gut. This disrupts the
receptor?s function, eventually killing the insects.

But researchers say some insects always survive the ordeal and over
time subsequent populations could develop resistance to the toxin. Luthe and
her colleagues Srinidhi Mohan and Peter W. K. Ma, of Mississippi State
University and W. Paul Williams of US Department of Agriculture studied a
unique plant-based insecticide known as Mir1-CP.

Their goal is to see if Mir1-CP, when used in tandem with other
biological pesticides, such as the Bt toxin, can prevent pests from
developing resistance and make the toxin more effective.

?This protein, which we developed from certain strains of corn from
Antigua, breaks down other proteins and peptides in the insects? gut,? said
Luthe.

Unlike Bt, Mir1-CP breaks down proteins in a protective membrane
covering the mid-gut. This membrane acts as a barrier that protects the
caterpillar from toxins in the diet, and cycles nutrients to the mid-gut.
?It is the caterpillar?s first line of defence against toxins and chemicals
in its diet,? said Luthe.

They found that when used alone, a concentration of Bt at five parts
per billion killed four percent of all corn earworms and five percent of
tobacco budworms. Mir1-CP, when used at a concentration of 60 parts per
billion, killed eight percent of the corn earworms and 3 percent of the
tobacco budworms.

But when researchers added the two insecticides together, the mixture
killed 61 percent of corn earworms and 57 percent of tobacco budworms, which
is more than 10 times better than either by itself.


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